Ellipsometer Focusing System with Beam Splitter
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Solution Overview
Problem
Conventional ellipsometers face challenges in achieving precise focusing due to susceptibility to inaccuracies from stray light and systematic errors, requiring expensive and difficult alignment, and lacking the precision to produce a small illuminating spot size.
Innovation Solution
An integrated focusing system with a beam splitter that magnifies deviations from the best focus position, utilizing a two-dimensional sensor and a compensator to correct optical aberrations, and a processor to adjust the focal position, along with a proportional-integral-derivative controller for exposure control, to enhance precision and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an independent focusing system with a mirror and aperture is used, then the focusing position can be determined, but the system requires very precise alignment which is expensive and difficult
Solution Approach 1:
The patent combines the focusing detection function with the main optical path by using a beam splitter to direct a portion of the reflected light to a camera sensor. This integration eliminates the need for separate independent focusing systems with mirrors and apertures, reducing alignment complexity while maintaining focusing position determination capability
Solution Approach 2:
The beam splitter acts as an intermediary element that divides the reflected light beam, directing part of it to the camera for focusing detection while allowing the rest to continue to the ellipsometer detector. This mediator enables focusing measurement without requiring complex independent optical paths
2Measurement precision
If a position sensitive device or quad cell is used as focus detector, then the focus position can be detected, but the device is susceptible to inaccuracies due to stray light
Solution Approach 1:
The patent uses a camera sensor to capture an image of the focused light spot, creating a visual copy of the focal point. This image-based approach allows for more reliable spot center determination compared to position sensitive devices, as the entire spot profile can be analyzed to identify the true center, making the system less susceptible to stray light inaccuracies
Solution Approach 2:
The patent transitions from one-dimensional position detection (as in PSD or quad cell) to two-dimensional image capture using a camera sensor. This dimensional change enables more robust focus detection by analyzing the spatial distribution of light across the entire spot, improving reliability against stray light interference
3Device complexity
If only the outer part of the beam is sampled by the detector, then the focusing system can be simpler, but systematic errors are caused reducing precision
Solution Approach 1:
The beam splitter enables the system to sample the entire reflected beam for focusing detection while simultaneously maintaining the full optical path for ellipsometer measurements. This multi-functional approach allows complete beam utilization for focus detection without compromising measurement precision, unlike systems that only sample the outer beam portion
4Ease of operation
If a larger spot size is used, then the focusing system can be more tolerant of alignment errors, but the precision required to produce a small illuminating spot size is not achieved
Solution Approach 1:
The patent implements an automatic focusing system where the camera captures the spot position, the processor determines deviation from the optimal focus position, and feedback is provided to adjust the focusing mechanism. This closed-loop feedback enables precise spot size control while maintaining alignment tolerance, as the system continuously corrects for alignment variations
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides high precision focusing with improved measurement accuracy, reduced susceptibility to systematic errors, and increased auto-focus range, enabling the use of small spot sizes for precise sample analysis.
Implementation Method 1
an integrated focusing system that includes a beam splitter in the beam path between the sample and the ellipsometer detector
Implementation Method 2
a lens system that magnifies any deviation from a best focus position by at least 2×
Implementation Method 3
a two-dimensional sensor, where the spot of light focused on the sensor is 50 percent or smaller than the sensor
Implementation Method 4
a compensator positioned between the beam splitter and the detector to corrects optical aberrations caused by the beam splitter
Data Source
AI summary
An ellipsometer includes an integrated focusing system with a beam splitter between the sample and the ellipsometer detector. The beam splitter provides a portion of the radiation to a lens system that magnifies any deviation from a best focus position by at least 2×. The focusing system includes a 2D sensor, where the spot of light focused on the sensor is 50 percent or smaller than the sensor. The focusing system may further include a compensator to correct optical aberrations caused by the beam splitter. A processor receives an image signal and finds the location of the spot from which focus error can be determined and used to correct the focal position of the ellipsometer. The processor compensates for movement of the spot caused by rotating optics. Additionally, a proportional-integral-derivative controller may be used to control exposure time and/or gain of the camera.